Method and device for transmiting data

ABSTRACT

A video consisting of data organized in the form of a plurality of images is transmitted in a communication network. The method comprises a step of coding images with motion compensation, which consists in compressing the images of the video and in creating dependencies between compressed images, a step of scheduling the transmission of packets representing the compressed images, which consists in sending the compressed images over the network in a selected order, and a step of controlling the rate of the video. At least one of reconsidering the selected order of sending already compressed but not yet transmitted images and deleting at least one compressed image is performed at the time of coding a new image. Furthermore, the dependencies between the new image to be compressed and the compressed images are selected by taking into account the reconsidered sending order at the time of coding the new image.

The present application claims priority of French patent application No.0854069 of Jun. 19, 2008, which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method and to a device fortransmitting data between a server and at least one client in acommunication network, and also to a server which carries out such amethod.

It belongs to the field of multimedia data transmission in a packetcommunication network. It applies in particular to the transmission oflive video data, which involves the coding of the compressed video withmotion compensation and the scheduling of the packets of the video datastream, over a network such as the Internet or local networks of the IP(“Internet Protocol”) type.

BACKGROUND OF THE INVENTION

In a digital multimedia data server connected to a communicationnetwork, for example a digital video surveillance camera orvideo-conferencing camera which sends audio and video data, the data iscoded in a digital compression format and then stored locally in abuffer memory or storage buffer before being transmitted over thenetwork to one or more clients.

These clients receive and consume this data, for example by playing thevideo and the audio, as they are received. This is known as multimedia“streaming”.

These multimedia streams consist of data such as images or pictures,portions or slices of images or sound samples which exhibit thecharacteristic of having a limited useful life span, that is, this datamust imperatively be received and processed by the receiving peripheraldevice before a certain expiry time. This expiry time corresponds to theinstant at which the data is required to be displayed or played by theclient. After this expiry time, the data item becomes useless and ispurely and simply ignored by the client.

The multimedia data is compressed so as to be able to be sent over anetwork with limited bandwidth. This compression degrades the quality ofthe data, but is necessary in order to adapt the data to the bandwidthof the network. It is therefore important to make the best possible useof the bandwidth of the network so as to compress the data as little aspossible and have the best quality.

It is also necessary to avoid modifying the compression rate of themultimedia data too quickly. This is because the quality perceived bythe user for data with a rapidly varying compression rate is severelydegraded. In the case of a video, for example, the eye is very sensitiveto the changes in quality of the images. It is therefore important tohave smooth variations.

In addition, compression creates dependencies between the successivedata items. Therefore, if one data item is not received, the followingdata items are corrupted for a certain period of time. In the case ofvideo data, this is known as compression with motion compensation. Thevideo may be coded in accordance with one of the standards described inthe ITU-T recommendations H.263, H.264, or else MPEG-4.

These multimedia streams are transmitted over communication networksconsisting of interconnection nodes (routers, switches, etc.) so as toconvey the data packets coming from source devices to destinationdevices. They share these networks with other data streams (for examplefrom Internet browsing, a video game, the transfer of a file to aprinter, etc.). All these data streams may thus create congestion on thecommunication networks when they pass through the same network link ofinsufficient capacity. The surplus packets generally end up beingrejected by the interconnection node located at the entry to the link.

Traditionally, servers and clients use communication protocols whichimplement control mechanisms so as to avoid continually losing a largequantity of data in the event of congestion. They make it possible todetect the occurrence of a congestion phenomenon as a function of packetlosses, and they act on the transmission rate of the data stream inorder to reduce it or increase it, so as to be compatible with theoverall bandwidth of the network.

The congestion control mechanisms generally used in IP networks are ofthe TFRC (“TCP Friendly Rate Control”, IETF RFC3448) or AIMD (“AdditiveIncrease/Multiplicative Decrease”, IETF RFC2581) type. These algorithmsperiodically calculate the quantity of data that can be sent. Packetscorresponding to this quantity of data are then taken from the buffer ofdata that have already been coded, and are sent over the network.

The rate calculated by the congestion control may vary very rapidly andmuch more rapidly than the changes in the compression rate of themultimedia data. This may lead to a significant increase in the quantityof data in the buffer of data to be sent, and may therefore give rise tosignificant waiting times. Due to the limited life span of the datapackets, this wait may render certain packets unusable by the client,which, due to the dependencies between data, may also have a verysubstantial impact on the quality perceived by the user.

It is therefore desired to reduce the impact of a rapid change in theavailable bandwidth on the quality of the data.

The patent document WO-A-2004 064373 discloses a method for coding avideo using a reference image buffer containing a plurality of images,these reference images being used in the context of the aforementioneddependency between data. A number of techniques are presented forselecting the best reference to use. In particular, the coder receivesthe information about bandwidth and packet loss. It can adapt the sizeof the images to the bandwidth. In the case of a reduction in bandwidth,the coder can select an older reference image since it is of betterquality. In the case of packet losses, it can calculate the propagationof the error and avoid using as reference an image that has beenaffected by the error.

However, the described system has the disadvantage in particular of alack of reactivity in response to the variations in bandwidth.

The paper by Sang H. Kang and Avideh Zakhor entitled “Packet schedulingalgorithm for wireless video streaming”, Packet Video workshop 2002,describes a packet scheduler for sending videos. The sending rate of thepackets is adapted to the constraints of the network bandwidth. Theorder of the packets to be sent is adapted by the scheduler to thestructure of the video and to the expiry times of the images: the imagesof type I have a higher priority than the images of type P; the images Pat the start of a Group Of Pictures (GOP) have a higher priority thanthe images P at the end of a GOP since they have a greater number ofdependent images.

However, this solution is also not satisfactory in terms of thereactivity in the event of a change in bandwidth.

The patent document U.S. Pat. No. 6,141,380 discloses a method forcoding a video. The coder may decide to skip the next image, that is,not to code it. This decision is based on the quality of the video, inparticular on the motion, and on an estimate of the available bandwidth.

However, this method also does not offer sufficient reactivity to adaptthe coding and the transmission of data to the variations in bandwidth.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the disadvantages ofthe prior art by determining both how to code the data, which packets tosend and in which order to send them, in particular as a function of thechange in available bandwidth.

To this end, the present invention proposes a method for transmitting avideo consisting of a plurality of images in a communication network,this method comprising:

a step of coding images with motion compensation, which consists incompressing the images of the video and in creating dependencies betweencompressed images,

a step of scheduling the transmission of packets representing thecompressed images, which consists in sending the compressed images overthe network in a selected order, and

a step of controlling the rate of the video,

wherein at least one of reconsidering the selected order of sendingalready compressed but not yet transmitted images and deleting at leastone compressed image is performed at the time of coding a new image.

The invention makes it possible to adapt the size, that is, the rate ofthe video, and its quality to the available bandwidth: by deleting or bydelaying the sending of certain images that have already been coded bymodifying the sending order, the coder can retain a sufficient size forthe next image to be coded; furthermore, the coding of the new imagetakes account of the images that might not be received.

Deleting an already compressed image and sending a newly coded imageinstead has to main advantages. First, the client is provided with themost recent image, and second, the transmitted image is the best adaptedone to the actual available bandwidth, contrary to the alreadycompressed image.

In addition, the invention makes it possible to recover rapidly from asignificant reduction in bandwidth, or even a cutting-off of thenetwork, with a small loss of quality.

Furthermore, in the case of a reduction in the available bandwidth ofthe network, the invention makes it possible not to destroy packets butmerely to delay the sending thereof; thus, in the event of a rapidincrease in bandwidth after the reduction, the system can often retrieveand finally send the packets that were initially delayed.

In a preferred embodiment, the dependencies between the new image to becompressed and the compressed images are selected by taking into accountthe reconsidered sending order at the time of coding the new image.

In this embodiment, the step of controlling the rate controls the stepsof image coding and packets transmission scheduling by selectingsimultaneously the dependencies between the new image to be compressedand the compressed images and the order of sending already compressedimages at the time of coding the new image.

Thus, the order of sending the compressed images is determined at thesame time as the choice of the coding mode and the selection of at leastone reference image, when an inter coding mode is chosen, for the newimage to be coded.

In a particular embodiment, the dependencies between images and thesending order of the compressed images are selected as a function of theavailable bandwidth of the network, and/or the number of packets waitingto be sent and the content of the video.

Thus, the coding of the transmitted video can vary rapidly as a functionof the variations in bandwidth of the network. The bandwidth may bemeasured directly or deduced implicitly from the number of packetswaiting to be sent.

According to a particular feature, the method furthermore comprises astep which consists in evaluating the available bandwidth by means of amechanism for controlling the congestion on the network.

The congestion control mechanisms make it possible to obtain anindication of the bandwidth actually available on the network in thepresence of concurrent traffic, without requiring any hardware supporton the part of the network infrastructure.

According to a particular feature, the congestion control mechanism isof the TFRC (“TCP Friendly Rate Control”) or AIMD (“AdditiveIncrease/Multiple Decrease”) type.

These congestion control mechanisms use only information that is simpleto calculate: the packet losses detected by the client and thecommunication time between the server and the client.

According to a particular feature, the method furthermore comprises astep which consists of determining a quality for the video as a functionof its content, this quality taking into account the images that havenot been sent.

This makes it possible to optimize the quality of the video actuallyreceived and decoded by the client, and not the quality of the codedvideo.

According to a particular feature, the selection of the dependenciesbetween the new image and the compressed images consists in defining acoding mode for the new image with or without dependency.

The selection of a coding mode (intra code mode generating imageswithout dependency, for example of type I, or inter coding modegenerating images with dependency, for example of type P) is easy tocarry out and applies to many codecs, even old codecs such as those ofthe MPEG2 type.

According to a particular feature, when a coding mode with dependency isdefined, the selection of the dependencies between images furthermoreconsists in defining at least one possible reference image.

The selection of the dependencies can thus be much finer, which makes itpossible to obtain a better adaptation of the rate of the video to theavailable network bandwidth.

According to a particular feature, the deleting at least one compressedimage that have not been transmitted is performed after a reordering ofthe compressed images.

The deletion of packets representing the compressed images is verysimple to carry out. Moreover, in some cases, packets must not be sentsince they would be incompatible with certain choices of changing thecoding of the video (for example, in the case of a deletion of an imageI).

The selection of the sending order of the packets may also consist indeciding to delay or bring forward the sending of at least onecompressed image.

It is thus possible to reserve the possibility of changing opinion ifthe bandwidth available on the network increases again. It will thus bepossible to send images which would not have been sent if the bandwidthhad remained low.

According to a particular feature, packets representing each compressedimage are assigned a certain priority and a certain expiry time, afterwhich the sending of the packets becomes useless, and the packets aresent in order of decreasing priority and in order of increasing expirytime.

This makes it possible to easily modify the order of sending of thepackets.

According to a particular feature, the video is coded in the H.264format.

This format, which is the most recent and the most effective to date,makes it possible, in particular, to have a plurality of referenceimages, which facilitates the dynamic selection of the reference images.

For the same purpose as that indicated above, the present invention alsoproposes a server for transmitting a video consisting of a plurality ofimages in a communication network, said server comprising:

a module for coding images with motion compensation, compressing theimages of the video and creating dependencies between compressed images,

a module for scheduling the transmission of packets representing thecompressed images, adapted to send the compressed images over thenetwork in a selected order, and

a module for controlling the rate of the video,

wherein at least one of reconsidering the selected order of sendingalready compressed but not yet transmitted images and deleting at leastone compressed image is performed at the time of coding a new image.

According to a preferred embodiment, the dependencies between the newimage to be compressed and the compressed images are selected by takinginto account the reconsidered sending order at the time of coding thenew image.

Still for the same purpose, the present invention also relates to atelecommunications system comprising a plurality of terminal devicesconnected via a telecommunications network, noteworthy in that itcomprises at least one terminal device equipped with a transmissionserver as briefly described above.

Still for the same purpose, the present invention also relates to ameans for storing information that can be read by a computer or amicroprocessor holding instructions for a computer program, noteworthyin that it makes it possible to carry out a transmission method asbriefly described above.

Still for the same purpose, the present invention also relates to acomputer program product which can be loaded onto a programmableapparatus, noteworthy in that it comprises sequences of instructions forcarrying out a transmission method as briefly described above, when thisprogram is loaded and executed by the programmable apparatus.

Since the particular features and the advantages of the transmissiondevice, of the telecommunications system, of the information storagemeans and of the computer program product are similar to those of thetransmission method, they are not repeated here.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and advantages of the invention will become apparent onreading the following detailed description of particular embodiments,given by way of non-limiting examples. The description refers to theaccompanying drawings, in which:

FIG. 1 schematically shows a data communication network of thedistributed type, capable of implementing the present invention;

FIG. 2 schematically shows a particular embodiment of a transmittingdevice adapted for implementing the present invention;

FIG. 3 schematically shows the architecture of a server capable ofimplementing the present invention, in a particular embodiment;

FIG. 4 illustrates the structure of a video that has been compressed forthe transmission thereof according to the present invention, in aparticular embodiment;

FIG. 5 is a graph illustrating a non-limiting example of congestioncontrol implemented in the context of the present invention;

FIG. 6 illustrates the principle of rate control implemented in thecontext of the present invention, in a non-limiting example;

FIG. 7 is a flow chart illustrating the main steps of scheduling packetsaccording to the present invention, in a particular embodiment;

FIG. 8 is a flow chart illustrating the main steps of controlling therate according to the present invention, in a particular embodiment;

FIGS. 9 a, 9 b and 9 c are flow charts illustrating the main steps ofselecting the coding mode according to the present invention, in aparticular embodiment; and

FIG. 10 schematically summarizes the different states and transitionsbetween possible states for images contained in a reference image buffermemory and in a packet buffer memory, when simulating the operation ofthese memories according to the present invention, in a particularembodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an example of a data communication network in which thepresent invention can be implemented.

A transmitting device or server 101 transmits data packets of a datastream to a receiving device or client 102 via a data communicationnetwork 100.

The network 100 may contain interconnection nodes 103 and links 104which create pathways between the transmitting and receiving devices.

The interconnection nodes 103 and the receiving device 102 may rejectdata packets in the event of congestion, that is, if the receivingmemory is overflowing.

The network 100 may, for example, be a wireless network of theWiFi/802.11a or b or g type, or an Ethernet or Internet network.

The data stream supplied by the server 101 comprises video informationcoded with motion compensation.

The transmitting device 101 may be any type of data processing devicecapable of coding and supplying a data stream to a receiving device. Byway of non-limiting example, the transmitting device may be a streamingserver capable of supplying content to clients on demand, for exampleusing the RTP protocol (“Real-time Transport Protocol”) on UDP (“UserDatagram Protocol”) or DCCP (“Datagram Congestion Control Protocol”) orany other type of communication protocol.

The transmitting device may implement a congestion control algorithm ofthe type mentioned above, namely, TFRC or AIMD.

The receiving device 102 receives the data, decodes them and displaysthem with a low latency.

Both the transmitting device 101 and the receiving device 102 may be forexample of the type shown in FIG. 2, described below.

FIG. 2 illustrates in particular a transmitting device 101 adapted forincorporating the invention, in a particular embodiment.

Preferably, the transmitting device 101 comprises a central processingunit (CPU) 201 capable of executing instructions coming from a programread-only memory (ROM) 203 when the transmitting device is powered up,and also instructions relating to a software application coming from amain memory 202 after powering-up.

The main memory 202 is for example of the random access memory (RAM)type and operates as a working zone of the CPU 201. The capacity of theRAM memory 202 may be increased by an optional RAM connected to anextension port (not shown).

The instructions relating to the software application may be loaded intothe main memory 202 from a hard disk 206 or else from the program ROM203 for example. In general, a means for storing information that can beread by a computer or by a microprocessor is adapted to store one ormore programs, execution of which makes it possible to carry out themethod according to the invention. This storage means may or may not beintegrated in the device 101 and is optionally removable. The executionof the program(s) mentioned above may take place for example when theinformation stored in the storage means is read by the computer or bythe microprocessor.

The software application, when it is executed by the CPU 201, causes theexecution of the steps of the flow charts in FIGS. 7 to 9.

The transmitting device 101 furthermore comprises a network interface204 which allows it to be connected to the communication network 100.The software application, when it is executed by the CPU 201, is able toreact to requests from the client 102 received via the network interface204 and to supply data streams to the client 102 via the network 100.

The transmitting device 101 additionally comprises a user interface 205,consisting for example of a screen and/or a keyboard and/or a pointingdevice such as a mouse or an optical pen, for displaying information toa user and/or receiving inputs from the latter. This interface isoptional.

An apparatus which implements the invention is for example amicrocomputer, a workstation, a digital assistant, a mobile telephone, adigital camcorder, a digital camera, a video surveillance camera (forexample of the webcam type), a DVD reader, a multimedia server or arouter element in a network. This apparatus may incorporate directly adigital image recorder or, as an option, may be connected to variousperipheral devices such as, for example, a digital camera (or a scanneror any means for acquiring or storing images) connected to a graphicscard and supplying multimedia data to the apparatus.

The apparatus may also have access to multimedia data on a storagemedium (for example the hard disk 206) or else may receive a multimediastream to be processed, for example from a network.

FIG. 3 illustrates in greater detail the architecture of a server 101according to the present invention, in a particular embodiment.

The server has as input a video coming for example from a sensor 305.The sensor 305 is for example a camera.

The video is supplied to a video coder or codec 310 which codes thevideo in a known format, for example H.264, then stores the result in abuffer memory 325 in the form of packets which are ready to be sent.Throughout the rest of the text, this buffer memory will be referred toas the packet buffer 325.

As a variant, the server may receive from another network a video thathas already been coded, for example in the case of a home gatewayreceiving a television channel via the Internet. In this case, the codec310 transcodes the video in order to adapt its rate to the bandwidth ofthe home network, which is for example a wireless network. As in thefirst case, the data created is stored in the packet buffer 325.

The codec 310 stores a plurality of reference images in a buffer memory315. Throughout the rest of the text, this buffer memory will bereferred to as the reference image buffer 315. These images have beencoded then decoded and will be used to compress the subsequent images.

The codec 310 is controlled by a video rate control module 320. Themodule 320 determines the quantity of data that the coder can produceand the quality of the coded video. To do this, conventionally, the ratecontrol module 320 calculates the quantization step of the next image asa function of the content of the packet buffer 325 and the networkbandwidth information coming from a congestion control module 335. Therate control module 320 also selects a sub-portion of the referenceimages that the coder can use.

The packets stored in the packet buffer 325 are read and sent over thenetwork by a packet scheduling module 330 which is responsible forsending the packets. The packet sending algorithm is described in moredetail below in connection with FIG. 7. The packet scheduling module 330decides which packets to send each time it is called upon by thecongestion control module 335.

The congestion control module 335 calculates the bandwidth available onthe network and decides when packets can be sent. To carry out thesecalculations, the module 335 uses information received from the clientvia the network, in particular packet loss events, the Round-Trip Time(RTT) on the network and the display latency.

The display latency is the limit life span of a data item. Beyond thelatency, the data can no longer be used since the image has already beendecoded in order to be displayed. This information is conventionallytransmitted using the RTCP messages of the RTP protocol. The displaylatency may also be received during an initial phase of initiating thecommunication (for example during Real Time Streaming Protocol (RTSP)exchanges, IETF RFC 2326).

FIG. 4 shows the structures defined for the coding based on motioncompensation. A video 400 compressed by a coder based on motioncompensation (for example according to the standard H.264, MPEG-4 Part2, etc.) is composed of a set of images 401, 402, 403, etc. coded in aplurality of modes: without motion compensation, this is known as intraor I coding (image 401). The inter mode (e.g. image 402) is based onmotion compensation on the basis of a reference image. The image codedwith this mode is known as predicted image or P image. The bidirectionalor B mode (e.g. image 403) is based on motion compensation on the basisof two reference images.

A Group Of Pictures (GOP) is a sequence of images starting with an imageI which makes no reference to an image in a previous GOP.

The images are compressed in macroblocks (blocks of 16×16 pixels). Themacroblocks are grouped into slices 420. In the case of sending a videoover a network, each slice is placed in an RTP packet and sent over thenetwork to the receiver.

In one image there may be different types of macroblocks: in an image I,all the macroblocks are of type I, but in an image P the macroblocks areof type I or P, and in an image B they are of type I, P or B. Eachmacroblock may also use a different reference image in the referenceimage buffer 315.

The reference image buffer 315 consists of a set of images 414, 412,etc. of the video sequence which have previously been coded thendecoded. It makes it possible to store the images used as referencesduring the motion compensation.

During the coding, each macroblock coded in the inter mode first passesthrough a compression step which starts with a motion calculation. Inthe inter mode, a macroblock can be partitioned to a size 4×4 in theH.264 format. In the motion calculation step, each partition is comparedwith zones in reference images contained in the reference image buffer315. The zone selected is the one which most resembles the pixels to becoded. There are numerous algorithms which make it possible to obtain agood result quickly without carrying out an exhaustive search in all thereference images.

The reference image buffer is used both in the coder and in the decoder.For the coder it is used during motion estimation, and for the decoderit is used for motion compensation. Consequently, this buffer must beupdated in the same way on both the client and server side. This bufferis generally filled by following the principle of a FIFO queue. The newreference images (such as the image 412 in FIG. 4) are added at the endof the queue of the reference image buffer. When the queue is full, theimage at the front of the queue is deleted (image 414 in the example ofFIG. 4).

However, there are more complex cases in which commands for manipulatingthe buffer may be associated with images.

In the case of the H.264 format, the reference image buffer 315 has thepossibility of distinguishing between two types of memories: theshort-term memory and the long-term memory. The short-term memoryoperates on the principle of the FIFO queue, described above. Thelong-term memory makes it possible to retain an image for a longerduration. Commands present in the headers of the slices make it possibleto label an image as a long-term reference. To remove the images fromthe long-term reference image buffer, it is necessary to use an explicitcommand (“mmco”), which is also specified in the header of the slices.

Still in the case of the H.264 format, an image I of type IDR(“Instantaneous Decoding Refresh”: for example, the first image of aGOP) empties the reference image buffer. The IDR type is specified inthe form of a flag in the header of the image slices.

According to the present invention, the search for motion vectors islimited to a sub-portion of the reference images selected by the ratecontrol module 320 as a function of criteria associated with thenetwork. This functionality is described in detail below.

The difference between the exact value of the image and the imageportion selected by the motion calculation is calculated. This value isknown as the residue.

The residue (or the value of the image in the case of an intramacroblock) is then transformed block by block and subsequentlyquantized.

The quantization makes it possible to check the quantity of dataproduced (this is a compression with losses). The quantization parameteris selected by the rate control module 320.

The result of the quantization passes through an entropic coding (suchas a Variable Length Coding) slice by slice. The coded macroblocks arethus obtained, which are then grouped into slices and sent in RTPpackets.

Depending on the size of the video, on the type of image coded (I, P orB), on the quantization step and on the content of the images, the sizeand therefore the number of packets produced per image is highlyvariable: from a few packets to several hundred.

In the case of a video with 25 images per second, a new image is codedevery 40 ms.

A congestion control algorithm calculates the quantity of data sent ateach instant on the network. This is because if all the packets whichmake up an image were sent within a very short period of time, thiswould create congestion on the network. It is therefore necessary tosmooth the sending of the packets in order to avoid sending too manypackets simultaneously. It is also necessary to adapt to the state ofcongestion of the network by varying the number of packets that arebeing sent.

The most well-known congestion control is that of TCP which is of theAIMD type. This algorithm works by gradually increasing the rate of datasent as long as the client indicates that the packets are beingcorrectly received.

On average, TCP sends one more packet for each correct round-trip, whichgives a linear increase. When an error appears, this means that thenetwork is congested; as a reaction, the rate is divided by 2. This modeof operation is illustrated by the curve in FIG. 5, showing the rate asa function of time. When there are no losses, the rate increaseslinearly. The loss events (marked at 510, 512, 514, 516 on the drawing)cause a drop in the available bandwidth. Several loss events maysometimes occur very close together, resulting in very substantial rapiddrops.

Because of the transmission time of the packet and the time taken toreceive the information as to whether the packet has been correctlyreceived, AIMD has a behaviour that is closely linked to the Round-TripTime (RTT) of the network. In a local or short-distance network, the RTTtime may be very short, typically around one millisecond. There maytherefore be very rapid variations in network bandwidth compared to thespeed of the coder (for example, one image every 40 ms).

There may also sometimes be a complete interruption in communication,for example if, because of considerable congestion, all the receiptacknowledgement packets have been lost. In this case, the interruptionmay last for several hundred milliseconds.

Like the congestion control algorithm AIMD, an algorithm of the typeTFRC also works by using the packet loss events and an RTT calculation.It calculates the rate by following an equation which has to give a ratecomparable to that of TCP, but with more smoothed variations. However,it has been found that, even if the variations are less abrupt, theremay be very considerable variations over very short periods of time inthe case of a network with a short RTT. As in the congestion controlalgorithm AIMD, there may be interruptions of several hundredmilliseconds.

In both cases, the congestion control module 335 receives controlinformation from the client, allowing it to calculate the loss rate orthe loss events and also the RTT time.

By using this information, the congestion control module 335 calculatesan available network bandwidth. It can thus activate the packetscheduler 330 repeatedly by spacing apart each activation over time.Upon each activation, it indicates to said packet scheduler a number ofpackets to be sent so as to comply with the current bandwidth.

The bandwidth information is also sent to the rate calculation of thecoder so that the latter can use it to adapt the size of the nextimages.

The principle of rate control according to the invention will now beexplained on the basis of a non-limiting example shown in FIG. 6.

Consider the case in which the network bandwidth has been stable onaverage for a period of several images and in which the availablenetwork bandwidth suddenly decreases. The previous images (601, 602,603, 604 in the drawing) have been coded, resulting in compressed images611, 612, 613, 614 of different types (I for 613, P for the others) andhaving variable sizes.

The first image 611 has already been sent to the client and hastherefore been deleted from the packet buffer 325. The three otherimages are still in the packet buffer 325 at the time when the coder hasto code the new image. Since the four coded images are of type I or P,in the case of a reference image buffer 315 organized as a FIFO queuewith 4 places, they can also be stored in the reference image buffer ofthe coder.

However, in the case where the image 613 is an image I of type IDR, thishas the consequence of invalidating the previous images 601, 602 in thereference image buffer. This is shown by the hatching on the images 601and 602 in the drawing.

At the instant in question, the server has to code a new image 605.

In the prior art (case 1), the rate control will calculate aquantization step for the new image, then the coder will select for eachmacroblock one of the reference images for the motion calculation (forexample the most recent image 604). Since there is a considerable dropin bandwidth, the new coded image 620 will be considerably reduced insize and therefore severely degraded in terms of quality.

It can easily be seen that, in order to send all the previous images andtherefore empty the buffer, the packet scheduler will spend more timethan initially estimated by the rate control. The situation maytherefore occur again where the data of the new image 620 cannot besent. This poses a serious problem since any future image using it asreference would not be able to be decoded by the client.

An improved version of the rate control (not shown in FIG. 6) could takeinto account the state of the packet buffer 325 in order to calculate astate of the reference image in the reference image buffer 315. If itfinds that the new image 605 cannot be sent, it can then label it asinvalid in the reference image buffer, so as to avoid using it as areference for the next images.

This example shows the case where the state of a reference image in thereference image buffer 315 is modified.

There are a number of other possibilities if the rate control module 320is authorised to control the buffer of the images that have previouslybeen coded but have not yet been sent (packet buffer 325).

In case 2, the rate control can decide not to send an old image (forexample the image 614) of the packet buffer 325. The image 614 caneasily be deleted without any impact on the other images of the codedvideo since it is an image P on which no other image is dependent. Ifthis image 614 is not sent, the coder can allow itself to code the newimage 630 with a larger size and therefore a better quality than in case1 (image 620). The coder must avoid taking the image 604 as reference,since its coded version 614 will not be sent. In FIG. 6, the coderselects for example for one macroblock a reference on the image 603.

In order not to send the image 604, it is possible to simply delete allthe packets of the coded image 614 in the packet buffer 325. However, itis better to change its priority and not to delete it immediately. Thisis because if the bandwidth of the network increases subsequently, itmay be possible to send it in time, even if it is sent after the image630.

This example shows the simultaneous modification of the state of animage in the reference image buffer 315 and the packet buffer 325.

Another possible solution (not shown) would be to delete the image 612.In this case, the new coded image could use either the image 603 or theimage 604 as reference.

In order to choose between case 1 and case 2 or this latter solution, itis necessary to take into account not only the quality of each image butalso the impact on the quality of the video. For example, in the casewhere there is a change in scene between the image 602 and the image 603(which would also justify the I coding of the start-of-scene image 613),but not between the image 604 and the image 605, it is better to choosethe latter case (deletion of the image 602). This is because thedeletion of an image before the change of scene is less noticeable thana deletion of an image within a scene.

There may also be the case 3 in which the image 613 (of type I) is ofsuch a large size that, with the new bandwidth, it risks delaying allthe subsequent images too much. The following solution may therefore bepreferred in this case: the image 613 of type I is deleted as well asthe image 614 since the latter uses the image 603 as a reference andtherefore cannot be decoded if the coded image 613 is not received.

If the image 613 is not sent, the images 601 and 602 are not invalidatedin the reference image buffer 315. It is therefore necessary to changetheir state from invalid to valid.

The coder thus has more bandwidth for the new image 640 and selects areference image from those which are or have to be sent (611 and 612).

The impact on the quality of the video must be estimated in order toascertain whether this solution is indeed the best possible solution.The deletion of a plurality of images may significantly degrade thequality of a video in the case of a video with rapid movements. On theother hand, in the case of slow movements, a video can more easilywithstand the deletion of a plurality of images.

This example shows that the state of a reference image in the referenceimage buffer 315 may pass from invalid to valid as a function of thechanges in state of the images in the packet buffer 325.

A last case is shown at 650, in which it is preferred to delete all theimages of the packet buffer (612, 613 and 614) in order to code the newimage, either with reference to the image 601 or in mode I. Since allthe previous images have been deleted, there is a larger bandwidth forthe new image. This method may be used for example in the case where thenew image is a start of a scene and it is therefore preferred to deletethe images at the end of the previous scene so as to have a betterquality in the new scene.

The flow chart of FIG. 7 illustrates the packet scheduling algorithmused by the packet scheduling module 330.

The packet scheduler 330 is called upon by the congestion control module335 at certain instants with the command to send N packets (step 700).

During a first step 705, a current priority variable is set to highpriority.

Then, during a test 710, it is determined whether the number of packetsto be sent has been reached. If this is the case, the algorithm isstopped (step 715).

Otherwise (step 720), all the packets of the packet buffer 325 whichhave the current priority are selected.

The packet having the shortest expiry time is then selected (step 725).The expiry time of a packet is calculated as a function of the date ofcreation of the image associated with the packet, the time taken tocommunicate with the client and the latency of the client, that is tosay the delay that the client can accept. This time is determined in anegotiation phase prior to establishing a video stream, or receivedregularly with RTCP packets. It is important to take account of thismaximum time so as not to send packets which are too late and are thusno longer useful to the client. This is what takes place during the test740, which consists in determining whether the expiry time can be met,then in step 745 in which the packets which are too old are destroyedwithout being sent, before returning to step 725 for selecting packetson the basis of expiry time.

If, in step 725, no further packets exist with the current priority, thecurrent priority may be lowered (step 730). If the priority was alreadyat the lowest acceptable level, the algorithm can be stopped (step 735).Otherwise, there is a return to step 720 of selecting packets on thebasis of priority.

It should be noted that the algorithm described here makes it possibleto manage a plurality of priority levels, since it might be chosen tosend certain packets which are more important before others, dependingfor example on a criterion of importance for the visual effect. However,it is important that the packets of lowest priority are not sent, sincethese might conflict with other packets that have already been sent,because of the dependencies between images. Only the rate control candecide to increase the priority of a packet in order to send it, takingaccount of the constraints of dependency between images.

In the case where it is determined during the test 740 that the expirytime of the packet can be met, the packet is sent (step 750).

A test is then carried out in order to ascertain whether all the packetsof the image have been sent (test 755). If this is the case, the ratecontrol module 320 is notified in step 760, so that the latter takesaccount of this in its step of selecting the priorities of the images.

There is then a return to the test 710 in order to check whether thecorrect number of packets has been sent.

The flow chart of FIG. 8 illustrates the rate control algorithm, whichmakes it possible both to select the coding characteristics for the nextimage (quantization, reference image) and the priorities of the packetsin the buffer of packets to be sent.

During a first step 805, an attempt is made to predict the averagebandwidth for the following instants. Ideally, it would be desirable toknow the bandwidth that will be available up to the time or date ofcoding the next image. As described above, the rate of variation of thebandwidth may be very rapid compared with the rate of the images. Thebandwidth value calculated by the congestion control is more of aninstantaneous value which it is therefore preferable to average so as toobtain an average bandwidth starting from the last image. This value isused as a predictive value for the coming instants.

Then, during a step 810, the time required to empty the packet buffer325 is evaluated, taking account of the current size thereof.

Taking account of the display latency of the client, it is possible tocalculate the maximum date on which a packet has to be sent, and thus tocheck whether all the images of the packet buffer meet their expiry date(test 812). If this is not the case, a choice is made to delete certainimages by making a different choice of coding (step 825).

If it is determined during the test 812 that the time required to emptythe packet buffer 325 is acceptable, an attempt is then made (step 815)to calculate the size of the image and the correct compressionparameters so as to meet the rate targets and not to cause the outputbuffer to overflow while maximizing the quality of the video.

One conventional technique consists in using a rate/distortion model. Byway of non-limiting examples, known models used in the context of MPEGcompression are TMN-5 or TMN-8. These models are based on a quadraticlaw:

B _(frame) =a/q+b/q ²

where:

-   -   B_(frame) is the rate target for the current image,    -   q is the average quantization step used in the image,    -   a and b are parameters of the model estimated by linear        regression, based on the previous images of the same type (I, P        or B).

It is then necessary to check whether the quality of the video thusobtained is acceptable (test 820). For this, a criterion of continuityof the quality of the images may be considered.

If the quality of the next image is considerably degraded compared tothe previous images, then in step 825 other coding choices areevaluated. This step is described in detail below in connection withFIG. 9 a. The result of this step is a choice of reference images whichcan be used by the coder, the priorities for sending the packets and thequantization step of the next image. It may also be desired tore-evaluate the coding scenarios if there is a considerable increase inquality of the image. This is because it may then be possible perhaps tosend images which were of low priority.

As a function of these values relating to the coding, it is possible inthe next step 830 to update the reference image buffer 315 of the coderand the priorities in the packet buffer 325.

It is then possible to proceed to the coding of the next image (step835).

The flow charts of FIGS. 9 a, 9 b and 9 c illustrate the main steps ofsimultaneously selecting reference images and images to be sent.

This algorithm simulates the mode of operation of the reference imagebuffer 315 and of the packet buffer 325 so as to discover which imagescan be sent and which images can be used as references, the objectivebeing to have a video of the best possible quality.

The algorithm uses the dependencies (use as reference image) between theimages that have already been coded. There is therefore a graph ofdependency between the previously coded images. It is said that an imageA is dependent on another image B if at least one macroblock of A usesas reference a zone of B.

Firstly, it is useful to simulate the mode of operation of the referenceimage buffer of the client so as to know exactly its content, in orderto discover the images that are able to be sent. This is because thesending or not of an image may have a number of consequences on thereference image buffer of the client and therefore on the choice ofother images that can be sent:

the addition of an image to the reference image buffer adds an imagethat can be used as reference. The dependent images can thus be sent.Conversely, therefore, if a reference image is not sent, the dependentimages cannot be sent.

the decoding and the addition of an image may destroy images in thereference image buffer of the client:

-   -   on the one hand, because of the FIFO mode of operation of the        reference image buffer with a limited memory. The addition of an        image therefore deletes the oldest image;    -   on the other hand, because of the commands to manipulate the        images of the long-term type in the reference image buffer;    -   finally, because of the images of the IDR type, which empty the        reference image buffer.

The reference images thus deleted can no longer be used by other imageswhich might be subsequently decoded.

The simulation uses two simulated buffers with a plurality of states foreach image:

the state of the images in the simulation of the reference image buffermay be:

-   -   Present: the image is present in the reference image buffer of        the client and can be used;    -   Invalid: the image is present in the reference image buffer, but        is not correct (it has not been transmitted in full or it is        dependent on images which are not present);    -   Destroyed: the image has been deleted from the reference image        buffer of the client;    -   Absent: the image has not been placed in the reference image        buffer of the client.

the state of the images in the packet buffer may be:

-   -   Sent: the image has been sent;    -   Deleted: the image has not been sent;    -   Unknown: the state of the image has not yet been selected.

The states of the simulation of the two buffers evolve in a mannerassociated with one another: the states that an image may have in thesimulation are: Unknown/Absent (U/A), Sent/Present (S/P), Sent/Invalid(S/I), Sent/Destroyed (S/D), Deleted/Absent (D/A), Deleted/invalid(D/I). These states, and also the transitions between states, are shownin FIG. 10. This figure refers to various steps shown in the flow chartsof FIGS. 9 b and 9 c described below.

Initially, all the coded images are added starting from the last IDRimage. These images have the states Unknown and Absent.

As shown in FIG. 9 a, in step 905, firstly the state of the referenceimage buffer of the client is calculated on the basis of the imagesdefinitely sent. For this, use is made of the information supplied bythe scheduler in step 760 of FIG. 7.

From an initial known state (for example, the last IDR image sent, whichhas thus emptied the reference image buffer of the client), firstlythere is simulated the mode of operation of the reference image bufferof the client for each image received and the constraints which resulttherefrom.

The images are considered, not in the order of sending, but rather inthe order of decoding of the video sequence. For each sent image (seeFIG. 9 b):

the state of the image in the packet buffer is changed to Sent (step955);

the mode of operation of the reference image buffer of the client issimulated: the image received is added and changed to the Present state(step 960), and the state of the images already present, which have beendeleted because of an instruction from the received image, is changed tothe Destroyed state (step 965);

the images which are dependent on a destroyed image and which come afterthe sent image are changed to the Deleted and Absent state (step 970).If a dependent image was already in the Sent/Present state, it ischanged to the Sent/Invalid state. If it was in the Sent/Destroyedstate, it does not change state;

recursively, the Deleted/Absent state continues to be propagated for allthe images dependent on an image in the Deleted or Invalid state (step975). As in step 970, if a dependent image was already in theSent/Present state, it is changed to the Sent/Invalid state. If it wasin the Sent/Destroyed state, it does not change state.

At the end of step 905 in FIG. 9 a, the current state of the referenceimage buffer of the client is thus obtained on the basis of the sentimages.

It is then useful (step 910) to take into account the state created bythe image which is currently being sent. This is because, when the ratecontrol module 320 executes its algorithm, an image may have started tobe sent. It may be decided to stop this sending, but the client willstill receive the packets already sent and therefore will execute theassociated commands for managing the reference image buffer.

For the image currently being sent, the steps of FIG. 9 b are carriedout separately from step 955: the image remains in the Unknown state.

The initial state of the simulation is thus obtained.

One or more scenarios of images deleted from the packet buffer 325 arethen simulated.

In step 915, one or more images to be deleted are selected. Only theimages having the Unknown state can be selected. Use may be made of aheuristic method, for example selecting the images having the largestsizes (this choice being particularly beneficial if there is aconsiderable drop in bandwidth or a long interruption, typically ofaround several hundred milliseconds), or selecting only images whichhave no or few dependent images.

The next simulation step 920 consists in simulating the consequences ofthe choices. For each image that has been selected as not to be sent(see FIG. 9 c):

the state of the image in the packet buffer is changed to Deleted (step985). If the state is Present, this state is changed to Invalid sincethis is the image currently being transmitted: even if the sendingthereof were to be stopped, it will nevertheless be present in thereference image buffer of the client, but in an incomplete form;

all the dependent images are recursively changed to the Deleted/Absentstate. If a dependent image was already in the Sent/Present state, it ischanged to the Sent/Invalid state (step 990).

The sending of the remaining images is then simulated. All the imagesare again taken in the order of decoding of the video sequence. For eachimage:

if the state is Unknown,

-   -   if its size is smaller than the available size, its size is        added to the quantity of data sent and then its sending is        simulated according to FIG. 9 b;    -   if the size of the image is too large compared to the available        bandwidth, the image is not sent and its deletion is simulated        according to FIG. 9 c.

if the state of the image is Sent, its impact is recalculated byfollowing the algorithm of FIG. 9 b. This is because, during the firstsimulation (step 905), not all the sent images were yet known andtherefore the number of images destroyed in step 965 may have beenunderestimated;

if the image is already in the Deleted state, it is ignored.

A complete scenario has thus been calculated: all the images are in theSent or Deleted state.

It is then possible (step 925) to evaluate the quality of the resultingvideo by taking account of the quality of the images, the number andplacement of the deleted images relative to the content of the video,the quantity of motion in the video and the changes in quality in thesuccessive images.

A value may for example be calculated as follows:

an average quality for the sequence is calculated: the average of thequalities of the images in the Present and Valid state. This gives afirst evaluation of the quality;

all the images are then reviewed starting from the oldest in the packetbuffer:

-   -   it may be estimated for each image in the Deleted/Absent state        whether it has a high or low visual impact depending on the        movements and changes of sequence: an image in a sequence with        considerable movement has a high impact, an image at the start        or end of a sequence or in a sequence with little movement has a        low impact;    -   an image in the Sent/Invalid state has a high impact on the        quality of the sequence: this is because the decoder risks        having great difficulty in correcting the visual effect;    -   finally, a sudden change in the level of compression for an        image in the Sent/Present state has a medium visual impact.

The impact calculated for each image makes it possible to modify thequality calculated for the sequence and to obtain a quality for thescenario.

The scenario thus created is stored (step 930). It is then possible toexplore other hypotheses (test 935) by repeating step 915 with otherchoices, which makes it possible to test a number of heuristic methodsor, if there is enough time, to simulate all the possible scenarios.

After having explored all the hypotheses, the best quality scenario canbe selected (step 940). The scenario contains the state of the referenceimage buffer and of the packet buffer: the images in the Present statecan be used in the reference image buffer of the coder; the others arein the Invalid or Absent state and must not be used as reference. Theimages in the Sent state which are still in the packet buffer have ahigh priority; the others have a low priority since they do not have tobe sent.

1. A method for transmitting a video consisting of a plurality of imagesin a communication network, said method comprising: a step of codingimages with motion compensation, which consists in compressing theimages of the video and in creating dependencies between compressedimages, a step of scheduling the transmission of packets representingthe compressed images, which consists in sending the compressed imagesover the network in a selected order, and a step of controlling the rateof the video, wherein at least one of reconsidering the selected orderof sending already compressed but not yet transmitted images anddeleting at least one compressed image is performed at the time ofcoding a new image.
 2. A method according to claim 1, wherein thedependencies between the new image to be compressed and the compressedimages are selected by taking into account the reconsidered sendingorder at the time of coding the new image.
 3. A method according toclaim 2, wherein the dependencies between images and the sending orderof the compressed images are selected as a function of the availablebandwidth of the network, and/or the number of packets waiting to besent and the content of the video.
 4. A method according to claim 3,wherein it furthermore comprises a step which consists in evaluatingsaid available bandwidth by means of a mechanism for controlling thecongestion on the network.
 5. A method according to claim 4, wherein thecongestion control mechanism is of the TFRC (“TCP Friendly RateControl”) or AIMD (“Additive Increase/Multiple Decrease”) type.
 6. Amethod according to claim 2, wherein the selection of the dependenciesbetween the new image and the compressed images consists in defining acoding mode for the new image with or without dependency.
 7. A methodaccording to claim 6, wherein, when a coding mode with dependency isdefined, said selection of the dependencies between images furthermoreconsists in defining at least one possible reference image.
 8. A methodaccording to any one of the preceding claims, wherein the deleting of atleast one compressed image that have not been transmitted is performedafter a reordering of the compressed images.
 9. A method according toclaim 1, wherein the selection of the order of sending of the packetsconsists in deciding to delay or bring forward the sending of at leastone compressed image.
 10. A method according to claim 1, wherein thepackets representing each compressed image are assigned a certainpriority and a certain expiry time, after which the sending of thepackets becomes useless, and in that the packets are sent in adecreasing priority order and in an increasing expiry time order.
 11. Amethod according to claim 1, wherein the video is coded in the H.264format.
 12. A server for transmitting a video consisting of a pluralityof images in a communication network, said server comprising: means forcoding images with motion compensation, compressing the images of thevideo and creating dependencies between compressed images, means forscheduling the transmission of packets representing the compressedimages, adapted to send the compressed images over the network in aselected order, and means for controlling the rate of the video, whereinat least one of reconsidering the selected order of sending alreadycompressed but not yet transmitted images and deleting at least onecompressed image is performed at the time of coding a new image.
 13. Aserver according to claim 12, wherein the dependencies between the newimage to be compressed and the compressed images are selected by takinginto account the reconsidered sending order at the time of coding thenew image.
 14. A server according to claim 13, wherein it is adapted toselect the dependencies between images and the order of sending of thecompressed images as a function of the available bandwidth of thenetwork, and/or the number of packets waiting to be sent and the contentof the video.
 15. A server according to claim 14, wherein it furthermorecomprises means adapted to evaluate said available bandwidth by means ofa mechanism for controlling the congestion on the network.
 16. A serveraccording to claim 15, characterized in that the congestion controlmechanism is of the TFRC (“TCP Friendly Rate Control”) or AIMD(“Additive Increase/Multiple Decrease”) type.
 17. A server according toclaim 13, wherein it furthermore comprises means adapted to define acoding mode for the new image with or without dependency.
 18. A serveraccording to claim 17, wherein said means adapted to define a codingmode with dependency is adapted to define at least one possiblereference image.
 19. A server according to any one of claims 12 to 18,wherein it comprises means adapted to decide to delete at least onecompressed image that have not been transmitted after a reordering ofthe compressed images.
 20. A server according to claim 12, wherein itfurthermore comprises means adapted to decide to delay or bring forwardthe sending of at least one compressed image.
 21. A server according to12, wherein it furthermore comprises means adapted to assign to packetsrepresenting each compressed image a certain priority and a certainexpiry time, after which the sending of the packets becomes useless, andwherein it is adapted to send the packets in a decreasing priority orderand in an increasing expiry time order.
 22. A server according to claim12, wherein the video is coded in the H.264 format.
 23. Atelecommunications system comprising a plurality of terminal devicesconnected via a telecommunications network, wherein it comprises atleast one terminal device equipped with a server according to any one ofclaims 12 to
 18. 24. Means for storing information that can be read by acomputer or a microprocessor holding instructions for a computerprogram, characterized in that it makes it possible to carry out atransmission method according to any one of claims 1 to
 7. 25. Acomputer program product which can be loaded onto a programmableapparatus, comprising sequences of instructions for carrying out atransmission method according to any one of claims 1 to 7, when thisprogram is loaded and executed by the programmable apparatus.